Because of potential danger to the environment, particularly resulting from acid rain, much attention has recently been devoted to the control of emissions from fossil fuel combustion. Attention has been directed especially to systems which permit retrofitting of existing installations to avoid the capital investment required to construct new facilities.
While a number of techniques have attracted attention, principal interest has focused on the direct injection of dry sorbent into the boiler and collection and disposal of the spent sorbent and fly ash on a once through basis. The sorbent which has heretofore proven most attractive is limestone or calcium carbonate. It may be injected either with the fuel, with the combustion air, or downstream in the combustion gases. Injection equipment is well known and conventional. The sorbent reacts with the sulfur dioxide to form a sulfated sorbent which is separated in the electrostatic precipitation equipment normally available in power plants. The increase in the mass of solids to be separated by the precipitation or other separating means is not unreasonable, provided an active sorbent is used. It has been reported by the Environmental Protection Agency, that limestone injection multistage burner technology is capable of cutting the marginal costs of sulfur dioxide emissions by one-half to two-thirds, or even more in those boilers that could be retrofit in accordance with the technology. The two other alternatives; namely head end coal desulfurization, or back end flue gas desulfurization, require substantial new capital expenditures and their installation would require substantial shut down times of the boiler.
The requirements of an acceptable sorbent are that it is (1) widely available, (2) low in cost, and (3) has a high capacity for neutralizing sulfur dioxide either by direct reaction, or by sorption. Limestone meets these qualifications quite well, but is not completely satisfactory for a number of reasons. One disadvantage of limestone is its lack of uniformity. Another disadvantage of using limestone as a sorbent is that calcined limestone normally has no more than about 50 percent capacity for conversion of sulfur dioxide to calcium sulfate. Furthermore, the reactivity and capacity of limestone pellets for sulfur dioxide is drastically reduced above 850.degree. C. (1562.degree. F.) because the pore structure of the calcined limestone is adversely affected at this temperature. As a result, the calcined limestone particles usually allow only shell loading of sulfur dioxide, and the central core of the pellets is unavailable for reaction.
A further disadvantage is that the calcium oxide content and uniformity of limestone is not satisfactory. The theoretical content of calcium oxide in limestone or calcium carbonate is 56 percent. Even high grade natural limestone (Calcite, Greer limestone, Vicron), because of other impurities, contains only about 80 percent calcium carbonate, thus, the active calcium oxide content is only about 45 percent. Moreover, the reactivity of natural limestone may vary from one quarry to another at various locations throughout the country. It is not economical to distribute the best grade of limestone from one quarry to utility plants at remote locations.
Natural limestone contains a relatively high percentage of impurities, including for example, Fe.sub.2 O.sub.3, SiO.sub.2, Al.sub.2 O.sub.3, and MgO. The reaction of these materials as well as the CaO with coal ash is probably a major cause for the reduction of the capacity of limestone to neutralize the sulfur dioxide.